This invention relates to methods and apparatuses for automatically and dynamically eliminating unwanted rotational phenomena occurring in a rotating body having mass, such as, dynamic imbalance and resonance, and in particular to systems which correct the dynamic balance and/or avoid resonance of said bodies while they are rotating in-place. As used herein the terms "rotational phenomena" and "rotational phenomenon" refer to phenomena which exhibit themselves in a body only when said body is rotating, such as, dynamic imbalance and resonance at critical rotational speeds.
It is well known that imbalance is an unwanted rotational phenomenon occurring in a rotating body relative to its axis of rotation. It is one of the leading causes of deterioration and vibration of rotating machinery. Rotating machinery that is kept in continuous dynamic balance will operate smoothly, and thereby greatly diminish material fatigue. This invention presents a system that provides for dynamic adjustments to the balance of rotating bodies on an as needed basis, that is, the adjustments are made while the bodies are in motion whenever an imbalance is detected. The adjustments are made automatically in response to sensors in a closed or open loop control system. This invention is usable on a wide variety of rotating machinery and requires no operator intervention.
The necessity of maintaining dynamic balance in rotating machinery is well known. Modern high speed rotating machines require precision balancing, but heat and other environmental conditions make it difficult to maintain precision balance. These conditions are exacerbated by any small degree of unbalance, i.e. a small amount of unbalance induces greater wear in the bearings supporting the shaft and this greater wear induces more unbalance until failure of the entire rotating machine may take place. The problem can again be exacerbated by vibrations, resonating parts, and high centrifugal forces generated by the unbalanced mass which can cause the shaft to whirl. Until now no effective means of automatically and dynamically correcting for unbalance, other than for some specialized systems, has been proposed. One such specialized system is the freon balancer that is used in conjunction with grinding wheels, i.e. by converting the fluid to vapor and by transfer of fluid to vapor, balance is achieved. Most automatic dynamic balancing systems, like the freon balancer, are specially designed for a specific machine only and not generally adaptable to other types of rotating machinery.
Except for a very few specific devices, there is no general method for providing continuous dynamic balance. U.S. Pat. No. 4,458,554 fly wheels for space vehicle balance and U.S. Pat. No. 4,530,239 presents a ground operator "in hand" balancing.
In contrast to static unbalance which can be corrected by adding or subtracting force in a single radial plane, that is, a plane normal to the axis of rotation, dynamic unbalance presents many difficulties. A rotating body may be in static balance and not in dynamic balance, i.e. an unbalancing couple cannot be compensated for by adding or subtracting a force in a single plane. A compensating couple must be added or subtracted from the rotating body to make the geometric centerline coincide with the mass centerline of the rotating body, so two forces in two radial planes, at least, must be added to or subtracted from the rotating body to get dynamic balance. In some cases, such as dynamic balancing of large turbo-rotors, a three-plane balancing system may be required. In the process of achieving dynamic balance, any corrective force added or subtracted in one plane affects the other plane, so that the process of changing forces in each plane is an interactive one in which many attempts are made before dynamic balance is achieved.
New methods and programs for calculating weights to be added or subtracted in each plane have been developed in recent years. However, none of these procedures are truly automatic and require a manual intervention for their execution. For example, many procedures called "automatic dynamic balancing" are simply procedures for performing stationary in-place balancing which has the advantage of eliminating costly, time-consuming disassembly, and reinstallation of the rotor in the machine, but which nevertheless requires that the rotor be stopped and, further, requires manual intervention.
For example, there exists computer programs, such as the programs in a Model #245 Balancer made by IRD Mechanalysis, that automatically provide an operator with information as to an amount and location of correction weights to be added to at least two planes of a rotor to achieve dynamic balance. When an imbalance is detected, a human operator adds and removes a trial weight, and this is sufficient to enable the programs to compute the amount and location of the needed correction weights to be added to each plane for dynamic balancing. A computer read-out then communicates that information to the operator who then stops the rotation and adds the indicated weights at the indicated locations.
The automatic balancing apparatus presented herein is generally adaptable to many various types of rotating machinery. It provides for two-plane correction of rotor imbalance and can easily be adapted to provide three-plane or more corrections.
A further advantage of this invention is that it can also be used to avoid or reduce rotor vibrations due to another unwanted rotational phenomenon occurring relative to an axis of rotation, that is, resonance. It is well known that even perfectly balanced, rotating machinery suffers from vibrational problems due to resonance. Rotating machines have a shaft that is a rotating structural member and, as such, the shaft will have elastic deflection. No matter how finely dynamically balanced, some amount of imbalance will exist, because the center of mass and the geometric center do not exactly coincide. As a result of this dynamic imbalance, there will exist a centrifugal force which will cause the shaft to deflect. At critical rotational speeds the rate of deflection matches or closely matches a natural frequency of the body or a significant harmonic thereof, and resonance can occur which causes the deflections to become amplified and can result in dynamic instability. Resonance can occur whenever a forcing frequency, i.e., the frequency of a periodic force acting upon a body causing it to rotate, equals or very nearly equals a natural frequency of said body or a harmonic thereof. When resonance occurs, the body is set into vibration with a relatively large and often destructive amplitude.
Heretofore, resonance in rotating machinery has been avoided in most cases by controlling the frequency of the driving force, i.e., by increasing or decreasing said driving force frequency to make it different from a natural frequency and its harmonics. This has the direct effect of increasing or decreasing, respectively, the angular velocity of the rotating body. However there are many situations where it is not always practical to change the angular velocity of a rotating body to avoid resonance. For example, in some helicopters, under certain conditions of wind gusts or maneuvers, flutter or other main rotor vibrations can occur, or resonant frequencies can be excited from the interaction of the forcing frequencies caused by the main and tail rotor oscillations. The shedding of vortices off the tips of the helicopter blades may also exacerbate the vibrational problems. In helicopters with gas turbine engines, it is desirable to maintain constant engine rotational speed under substantially all conditions. This invention provides the means and method for avoiding resonance without changing engine rotational speed. Rather than simply avoiding disturbing frequencies which are too close to the natural frequency of the rotating body and its harmonics, this invention provides a way to selectively change said natural frequency. Also, this invention provides the means to establish a variable dynamic energy absorber in a rotating system operating over a range of disturbing frequencies.
Other advantages and attributes of this invention will be readily discernible from the specification and claims hereinafter.